The top U.S. disease prevention official said Thursday that there could be "hundreds of thousands" of Zika cases in the U.S. territory of Puerto Rico in the coming months if precautions are not taken.

Dr. Tom Frieden, chief of the U.S. Centers for Disease Control and Prevention, told reporters he was making Puerto Rico his top priority in the fight against the virus.

"Nothing about Zika is going to be quick, and nothing about Zika is going to be easy," Frieden said. "We can't eliminate Zika at this point, but we can protect people and greatly reduce the impact."

The mosquito-borne virus is especially dangerous for pregnant women.

Frieden, who visited the Caribbean island this week, said that just two weeks ago, Puerto Ricans appeared unconcerned about Zika. That's not true now, he said.

"Every pregnant woman we talked with is taking this very seriously," he said. "They all have heard of Zika. They are all very worried about Zika. They want help and support to protect themselves against Zika."

Friden said he was especially concerned about the large number of women in Puerto Rico who said they did not want to get pregnant but did not use birth control. He said these women clearly should be using contraception.

Most Zika cases are mild, and many victims never show any symptoms. But the virus is suspected of causing pregnant women to give birth to babies affected by microcephaly, a defect that leaves them with unusually small heads and brains.

Brazil, which has been hit hardest by Zika, has reported more than 4,000 cases of microcephaly since October.

Health experts are strongly advising women who are pregnant or planning to become pregnant to avoid traveling to countries affected by Zika, including much of Latin America.